1. Field of the Invention
The present invention relates to an improvement of an impeller for an axial-flow blower used in various office automation equipment and the like.
2. Description of the Related Art
Office automation equipments, such as computers or copying machines, have many electronic parts enclosed in the housing thereof, generating excessive heat therefrom, which may destroy the parts. Accordingly, an axial-flow blower is mounted at a ventilation hole provided in the housing to release the interior heat to the outside therefrom.
This conventional type of blower, in this case an axial-flow blower driven by an outer rotor type motor, will be hereinafter described with reference to FIG. 10.
As shown in the drawing, a shaft 4 is rotatably penetrated and supported in a cylindrical section 1a at the center of a casing 1 via bearings 2 and 3.
The shaft 4 is mounted at the center of an impeller 5 (the center of a cup section 5a) including the cup section 5a and a plurality of impeller blades 5b at the outer periphery thereof.
The cup section 5a has a motor yoke 6a molded at the inner periphery thereof, to which a ring-shaped permanent magnet 6b is fixed. The permanent magnet 6b forms a principle structure of a rotor (an outer rotor) 6 together with the motor yoke 6a. 
The cylindrical section 1a has a stator 7 including a stator core 7a and windings 7b opposing the permanent magnet 6b fixed to the outside thereof. A printed circuit board 8 on which an electronic circuit is mounted for supplying a predetermined current to the stator windings 7b to operate the stator 7 and the rotor 6 as a stator and a rotor of a brushless DC motor is attached below the stator 7.
The stator winding 7b is connected to the electronic circuit on the printed circuit board 8 with a pin 9. The printed circuit board 8 is connected to a lead wire 10.
In the blower thus constructed, when a power of a predetermined DC voltage is applied to the lead wire 10, a current controlled by the electronic circuit on the printed circuit board 8 flows in the stator winding 7. Accordingly, magnetic flux is generated from the stator core 7a to rotate the rotor 6 around the shaft 4 by the mutual magnetic action with magnetic flux from the permanent magnet 6b, thereby rotating the impeller 5 integrated with the motor yoke 6a of the rotor 6 to blow air.
In this case, the impeller 5 is often made by synthetic resin molding. FIGS. 11 and 12 are a rear view and a partly cutaway left side view, respectively, showing the rotor section of the axial-flow blower including such an impeller 5. In each drawing, same numerals are shown if same with or similar to parts indicated in FIG. 10.
In the case where the impeller 5 of the axial-flow blower as shown in the drawings is formed by synthetic resin molding, an axial coupling type die is used. Accordingly, as shown in FIG. 11, one of the forming conditions is that when the impeller 5 is seen from the axial direction, adjacent impeller blades 5b and 5b among the impeller blades 5b arranged at the outer periphery of the cup section 5a at equal angles are set so as not to overlap each other.
In such an axial-flow blower, there is a case in which it is required that the adjacent impeller blades 5b and 5b overlap each other when looking at the impeller 5 from the axial direction for reasons such as increase of static pressure.
However, according to the conventional art, it is not easy to form such an impeller 5 by synthetic resin molding at low cost because of the above forming conditions, whereby improvements thereto have been required.
Also, in the case where the axial-flow blower is mounted in office automation equipments, which are located in a relatively quiet place, it has been strongly required to take measures to reduce noise generated during the rotation of the impeller (blower).
The present invention is made in light of the above problems. Accordingly, it is an object of the present invention to provide an impeller of an axial-flow blower having a structure in which adjacent impeller blades overlap each other when seen from the axial direction in an easy procedure and at a low cost even though by synthetic resin molding. In addition to the above, noise generated during rotation can be also reduced.
In order to attain the above object, according to the present invention, there is provided an impeller of an axial-flow blower driven by an outer rotor type motor and integrated with a rotatably supported shaft and a motor yoke, the impeller and the motor yoke rotating outside the stator around the shaft to blow air, wherein the impeller comprises a plurality of individual impeller sections each having a different number of impeller blades, the individual impeller sections being separately formed of a synthetic resin and arranged in series in the axial direction of the shaft.
In the present invention, preferably, among the plurality of individual impeller sections that are adjacent with each other at the front and rear sides in the axial direction of the shaft, the number of the impeller blades of the front section impeller is more than that of the rear section impeller.